1. Field of the Invention
The present invention relates to the preparation of ferric chloride from dilute hydrochloric acid, and, more especially, to the preparation of 35% to 45% by weight aqueous solutions of ferric chloride.
2. Description of the Prior Art
The above aqueous solutions of ferric chloride are known to be useful as flocculating agents in water treatment. Compare, for example, Kirk-Othmer, Encyclopedia of Chemical Technology, 3rd edition, volume 24, pages 394-396 (1984) and volume 10, page 498 (1980).
The simplest process for the preparation of ferric chloride solutions entails digesting iron with concentrated hydrochloric acid; a solution containing approximately 36% by weight of ferrous chloride (FeCl.sub.2) is thus obtained, and this is chlorinated to produce an aqueous solution of ferric chloride (FeCl.sub.3) analyzing, by titration, at approximately 41% by weight. This 41% solution can be directly employed as a flocculating agent, and is the usual commercial form. A concentrated solution of FeCl.sub.2 must be produced because FeCl.sub.2 and FeCl.sub.3 can undergo a partial hydrolysis over the course of concentration by evaporation, to give HCl. The presence of HCl in FeCl.sub.3 is a particular problem in water treatment. The above process also requires the use of concentrated hydrochloric acid.
U.S. Pat. No. 4,066,748 describes a process for preparing ferric chloride beginning with a solution of FeCl.sub.2 emanating from a descaling bath. This process requires both a concentration of the ferrous chloride as well as a two-step chlorination.
U.S. Pat. No. 3,682,592 describes a process similar to that of the '748 patent, but in which the ferrous chloride solution is contacted with oxygen.
Serious need exists in this art for a simple process for preparing ferric chloride from dilute hydrochlorine acid. A notable solution of hydrochloric acid is the water/HCl azeotrope, at a concentration of 20.5% by weight at atmospheric pressure. However, to produce ferrous chloride solutions containing a least 36% by weight FeCl.sub.2, it is necessary to begin with an approximately 24% by weight acid solution.